Mercedes-Benz C30 CDI AMG SC

AMG calls it the first-ever diesel sports car. Westfield, among others, might dispute that claim but the point is made. Mercedes' official tuning arm now embraces diesel power, ratcheting it a little further along the road of petrolhead cred.


Maybe that means there will be dieselheads in the future. Or maybe not. Meanwhile, there's a 3-litre C-class Sports Coupe with 398lb ft of torque to consider. Or a saloon, or an estate - but the Coupe is the one that looks most like the sports car of AMG's declaration.

This engine also delivers 231bhp, a popular German tax-break figure which just happens to be the same as that produced by BMW's similar-capacity petrol engine except that this peak happens at 3800rpm. But figures alone are not enough to make an engine excite an evocentric appreciation. It needs to tingle senses other than those that monitor the neck muscles, and that can be hard work for a diesel when hearing is the main sense in question.

But this is not as other diesels. It has a voice, a deep, slightly throbby five-cylinder note vocalised through two fat, gently downturned tailpipes, which takes on the dieselly rattle up front and mostly wins. Besides, it's not that rattly, even though there's a little more combustive clatter and thrum than in the regular C270 CDI from which this engine is descended.

That descent is more than just a longer stroke and extra fuelling. The iron block casting is new, with a cast-in oil rail to direct cooling oil under the piston crowns. The head has freer-flowing ports and water passages, and a lower compression ratio to handle the higher boost pressures. The head bolts are hollow, too, so cooling oil can flow through them to reduce their expansion and, therefore, maintain clamping force.

The other main engine innovation is a water-cooled intercooler with its own radiators. An intercooler usually relies on outside air passing through its fins to cool the induction air that the turbocharger forces through its channels, but here we have two remote radiators with their own water circuit and pump. The cooled water from these chills the intercooler's induction air to just the right amount, altered electronically by the pump depending on the engine's needs and the outside air temperature. This way, the engine can be taken more precisely to its boost limits without fear of damage. The engine electronics similarly modulate the turbocharger's variable vanes for optimum response.

And what a response it is. Just a tickle of the accelerator blips the engine into a low-inertia rev that's hard to square with the diesel soundtrack, and it's no false promise. With ESP switched out, it'll smoke its rear tyres from a standing start, then change up a gear for the next shove of g-force until all five gears are used up.

AMG claims a 6.8sec 0-62mph time, but that doesn't reveal the shoulder-shrugging ease with which the C30 will surge past slower traffic, pulling massively from 1800rpm or less. If you're ambling, an upshift might see the engine trickling down to 1500rpm and you expect the fire to go out, but it still blazes.

It does 155mph, of course, the AMG C30 CDI being easily the quickest SC of all. Even better, it averages an extraordinary 37.2mpg. It also has a neat trick unique to AMG autoboxes. There's Mercedes' usual not-quite-Tiptronic side-to-side manual override, but if you pull this one's lever to the left and hold it for a second it will automatically select the gear to give the best overtaking ability at that speed. So if you're waiting to blast past, you don't have to contend with the momentary pause of a kickdown - although that, too, is smooth and quick enough compared with a petrol C-class's sometimes abrupt shift. It also automatically holds a gear in a corner even if you ease the accelerator, and down-shifts as you slow so the engine gives its best when you accelerate again.

Beyond the enhanced powertrain, the C30 gets a chassis and body makeover much like that of the V6-engined C32, with recalibrated springs and dampers, stiffer anti-roll bars, a stronger differential and tougher driveshafts to take the torque. After the rubbery aloofness of the regular C-class's steering (the Sports Coupe being the worst-afflicted), the AMG's is a joy: precise, proportional, and informative. Why can't all C-classes be like this? And it's not as if the ride is ruined, despite 245/40 ZR17 tyres at the back.

You need to be skilled and/or brave to disengage the ESP, though. Ample as those tyres are, their traction is easily breached when 398lb ft pours through them to tip the AMG into a benign but insistent powerslide. You can still enjoy the C30's balance and responsiveness with the ESP on, because its intervention is subtle: it reassures rather than frustrates, as with all the best systems.
Here is arguably the sportiest diesel yet offered, a rich driving experience in its own right, and proof that a diesel performance car needn't be an automotive oxymoron. But could you bring yourself to buy one? It would be a more knife-edged decision than ever were AMG not fudging the right-hand drive issue, claiming it's difficult to do. Yet the regular C270 comes right-handed, and the UK is one of AMG's strongest markets. Come on, guys...